[c-nsp] ASR9000 queue-limit ? ms

Pshem Kowalczyk pshem.k at gmail.com
Tue Oct 15 15:55:22 EDT 2013

Hi Saku

On 15 October 2013 22:35, Saku Ytti <saku at ytti.fi> wrote:
> On (2013-10-15 22:04 +1300), Pshem Kowalczyk wrote:
>> 1. Voice - 2-3 ms (and probably a policer around 35-40% of link size)
>> 2. Video - up to 10 ms (assuming HD streaming, otherwise less)
>> 3. Everything else - about 5 ms
> Is there platform where you can configure this?

We have it done on ASR9k, ASR903, ME3600X. The actual numbers are
slightly different, but the principle holds.
As you said the ms are turned into actual bytes, depending on the
interface speed. The biggest advantage - I can use the same scheme
across different platforms and interface speeds.

> Consider you'd have 100Mbps connection with:
> 10% BE
> 90% EF
> Then you configure 5ms for EF, i.e. 90Mbps*0.005s = 56kB
> Now lets imagine during office hours offered rate is fairly close to that, but
> outside office hours offered load would flip otherway around, with only 10% of
> EF.
> Then you'd actually have 10Mbps * X = 56kB, where X would be 44.8ms.
> So you'd buffer EF during office hours expected 5m and outside office hours
> you'd buffer 45ms, which would probably break your EF.
> Maybe the example is rather extreme, but I think it's highly typical to
> observe different distribution of offered loads throughout the day. And as far
> as I know, all platforms where you specify temporal number for queue will be
> programmed as fixed bytes in HW.
> So if you really do have variable offered rates throughout the day, you
> actually need to make some compromise which will be acceptable in both
> scenarios, but not perfect in either.
> So I would recommend using bytes when defining buffer sizes, unless your
> platform actually can do temporal buffers as function of _offered_ rate.

I must admit - in our case we generally don't go below 1G connections,
so the amount of buffer is always somewhat bigger :-)
Also - particularly in case of EF - if we manage to fill in a 3 ms of
EF queue we're probably already breaking the SLAs for EF ( <1ms of
jitter ).

kind regards

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